1. Field of the Invention
The present invention relates to a light guide film, a backlight unit for an ultrathin liquid crystal display device and a mobile computer and a method for producing a light guide film.
2. Discussion of the Background
Liquid crystal display devices in widespread use have been in a backlight system where light emission is executed by illuminating a liquid crystal layer from the rear face. In this system, a backlight unit such as an edge-lit backlight unit or a direct-lit backlight unit is mounted on the underside of the liquid crystal layer. As shown in FIG. 6, such a backlight unit for an ultrathin liquid crystal display device 110 generally includes a top plate 111 disposed on the backmost face of a liquid crystal display unit, a reflection sheet 112 disposed on the front face of the top plate 111, a light guide plate 113 disposed on the front face of the reflection sheet 112, an optical sheet 114 disposed on the front face of the light guide plate 113, and a light source 115 that emits rays of light toward the end face of the light guide plate 113 (see Japanese Unexamined Patent Application, Publication No. 2010-177130). In the backlight unit for an ultrathin liquid crystal display device 110 shown in FIG. 6, rays of light that are emitted from the light source 115 and enter the light guide plate 113 propagate through the light guide plate 113. A part of the propagating rays of light exit from the back face of the light guide plate 113, are reflected on the reflection sheet 112 and enter again the light guide plate 113.
In mobile computers having such a liquid crystal display unit, in order to enhance its portability and user-friendliness, a reduction in thickness and weight is required, leading to a requirement also for a reduction in thickness of the liquid crystal display unit. In particular, in an ultraslim laptop computer referred to as Ultrabook (registered trademark) in which the thickness of the thickest part of its housing is no greater than 21 mm, it is desired that the thickness of the liquid crystal display unit is about 4 mm to 5 mm, and thus, further a reduction in thickness of the backlight unit for an ultrathin liquid crystal display device incorporated into the liquid crystal display unit has been desired.
In regard to the backlight unit for an ultrathin liquid crystal display device of such an ultraslim mobile computer, in addition to the backlight unit having the reflection sheet 112 disposed on the back face of the light guide plate 113 shown in FIG. 6, a backlight unit is also proposed in which a reduction in thickness is attempted, as shown in FIG. 7, by dispensing with the reflection sheet 112 shown in FIG. 6. The backlight unit for an ultrathin liquid crystal display device 210 shown in FIG. 7 includes a metal top plate 211, a light guide plate 213 overlaid on the front face 212 of the top plate 211, an optical sheet 214 overlaid on the front face of the light guide plate 213, and a light source 215 that emits rays of light toward the end face of the light guide plate 213, in which the front face 212 of the top plate 211 is finished by polishing and functions as a reflection surface. In this example, the rays of light that are emitted from the light source 215 and enter the light guide plate 213 propagate through the light guide plate 213, and a part of the propagating rays of light exit from the back face of the light guide plate 213, are reflected on the reflection surface 212 of the top plate 211, and enter again the light guide plate 213. Thus, in the backlight unit for an ultrathin liquid crystal display device 210 shown in FIG. 7, the front face of the top plate 211 corresponds to the reflection surface 212, and therefore the reflection surface 212 can serve as the reflection sheet 112 shown in FIG. 6. Therefore, the backlight unit for an ultrathin liquid crystal display device 210 dispenses with the reflection sheet 112, leading to achievement of a reduction in thickness of the liquid crystal display unit.
Moreover, in such an ultraslim mobile computer, since the thickness of the liquid crystal display unit falls within the above range, the average thickness of the light guide film is required to be no greater than about 600 μm. Thus, a polycarbonate having superior light guiding properties and a predetermined strength, or the like is used as a material for forming such a light guide film.